Probably the biggest game played at the moment is League of Legends, which if you haven't heard of is a HUGELY popular online game which is definitely worth a look into.

Other than that StarCraft 2 used to be one most people played, and I think a number of us still play it to a certain degree, people will often be up for a game if you ask. All the old popular games on Steam we play, Left 4 Dead, Counter Strike, I still play Team Fortress 2 on a regular basis, and you can catch me on Dota 2 if you have it for a few games occasionally